Edmond,
Would you please explain dragon sickness?
Essentially, it is the very act of succumbing to the lust of 'gold upon which a dragon has long brooded'.
That people would be bewitched and enchanted by dragon-hoards and thus take actions that would estrange them from their peers, in a similar manner to the effects of the One Ring. In effect a curse would be laid upon them for their actions.
The theme of cursed treasure had been a powerful narrative thread in earlier works by Tolkien, especially in the tale of 'The Nauglafring' and this theme owes much to the story of Fafnir's and Sigurd's treasure (also known as Andvari's Hoard and Das Rheingold) from the Volsunga Saga. The Volsung's treasure on the one hand (in Germanic and Norse myth) and the gold of the Rodothlim (Book of Lost Tales), the Silmarils and indeed the One Ring on the other (within Tolkien's Legendarium) indiscriminately bring doom to all their owners one by one, although a few like Bilbo, Beren and Earendil escape the curse's full effects.
This theme appealed greatly to Tolkien, so much so that it inspired one of his finest poems; The Hoard, also known as Iumonna Gold Galdre Bewunden, translated; ancient gold, entangled with enchantment. The poem tells of a wonderful treasure of gold and silver and jewels owned successively by Elves, a lone Dwarf, a Dragon and a hero who becomes king, all of whom perish miserably, leaving the hoard in the end lost forever, buried in a grassy mound. The poem also makes it clear that the original owners are chained to the hoard, possessed by their own possession.
There is another type of curse if you will, distinct from a curse laid upon a hoard of treasure, that of the dragon's gaze. A dragon's ability to directly manipulate the minds of those who look into his eyes and to individually curse any whose name he knows. In other tales, both Turin Turambar and his sister make the mistake of making eye contact with a dragon (Glaurung) and are beguiled and his knowledge of who they are enable him to craft specific curses that set both on the road to incest and suicide.
In terms of the Volsunga saga, there is no mention of whether Sigurd looks into Fafnir's eyes while the dragon is still alive, but he does initially give a false name to avoid Fafnir's dying curse. As for The Hobbit's hero; Bilbo wisely avoids meeting Smaug's gaze and giving his real name.
Despite all of this, relatively few succumb to dragon sickness in The Hobbit as published. Bilbo (very briefly), Thorin himself (who heroically throws off its influence during the Battle of the Five Armies
), most of Thorin's fellow Dwarves to a lesser degree and the Master of Lake Town at some later date.
Ultimately, Thror's recovered treasure brings prosperity and peace to the region in the hands of those who can resist the dragon sickness; Dain (who renews the Kingdom under the mountain) and Bard (who reestablishes and rebuilds Dale and eventually extends his realm all the way down to include the rebuilt Lake Town). Those who cannot resist meet with personal disaster (the Master of Lake Town's death from starvation does not harm Esgaroth's recovery though) in accordance with the established theme.
This forms a stark contrast with Tolkien's models, the Volsung hoard is lost, as is the gold of the Rodothlim and also the treasure guarded by Beowulf's dragon. Tolkien in The Hobbit creates a near-catastrophe followed by a happy ending appropriate to a fairy story, in keeping with his ideas of eucatastrophe.
Bilbo doesn't end up with a river of gold (although he does get a fair portion), but the gold is used instead of hoarded and makes his world a better place.